treewide: Make still dont* Variables are optional in most cases

Go beyond the obvious setup hooks now, with a bit of sed, with a skipped case:

 - cc-wrapper's `dontlink`, because it already is handled.

Also, in nix files escaping was manually added.

EMP

pkgs/development/libraries/qt-5/hooks/qmake-hook.sh

@@ -31,6 +31,6 @@ qmakeConfigurePhase() {
     runHook postConfigure
 }
 
-if [ -z "$dontUseQmakeConfigure" -a -z "$configurePhase" ]; then
+if [ -z "${dontUseQmakeConfigure-}" -a -z "$configurePhase" ]; then
     configurePhase=qmakeConfigurePhase
 fi